This invention relates to titanium alloys and has particular reference to titanium alloys intended for use in conditions of high temperature and stress, particularly in aircraft engines.
Alloys have been proposed for use where service temperatures of up to 540.degree. C. are encountered. It will be appreciated that the alloys do not run at such service temperatures throughout the entire time the engine is operating. The maximum temperatures developed in an engine are normally believed to exist when the engine is operating from high airfields in high temperatures during the summer under conditions of maximum load. When the engine is operating in a cruise condition at high altitudes the engine will operate at much lower temperatures. However, the engine has to be designed with the so-called hot and high conditions taken into account. It is essential, therefore, that the alloys used in the engines are capable of withstanding high temperatures even if it is not necessary that they can withstand such high temperatures for thousands or tens of thousands of hours.
In British Patent Specification No. 1 208 319 there is described the alloy containing 6% aluminium, 5% zirconium, 0.5% molybdenum, 0.25% silicon, balance titanium. The alloy is suitable for use where service temperatures of up to 520.degree. C. are encountered. Further developments in alloy technology are described in British Patent Specification No. 1 492 262 which describes the alloy titanium, 5.5% aluminium, 3.5% tin, 3% zirconium, 1% niobium, 0.25% molybdenum, 0.3% silicon. Such an alloy is capable of operating satisfactorily at service temperatures of up to approximately 540.degree. C.
The alloy described in this latter patent is the most advanced near alpha alloy which is capable of being used in the welded condition. By the term "weldable" as used in the present context is meant that articles manufactured from the alloy can be used in the welded condition. It is not sufficient merely to be able to stick two pieces of metal together. The alloy in the post welded condition after suitable heat treatment must have properties virtually indistinguishable from the alloy in the pre-welded condition and the welding must not introduce a zone of weakness into the structure, which would be a cause of possible failure in the aircraft engine.
Increasing concern at fuel costs is leading to the development of aircraft engines which are increasingly fuel efficient. A basic method of increasing fuel efficiency is to increase the operating temperature of the engine and to reduce its weight. This has meant that titanium is being considered for use nearer the centre of the engine, where the operating temperatures are in any case higher, and also the overall operating temperature of the engines is being increased. These developments have led to a requirement for a titanium alloy capable of operating at service temperatures of up to 600.degree. C. It will be appreciated that to produce titanium alloys having such high service temperatures is extremely difficult. The commercial development of titanium alloys for aircraft engines is only some thirty years' old and the titanium technology is as yet an incompletely understood science. In the past increases in service temperatures of 10.degree. or 20.degree. C. have been the maximum which have been obtainable. To move, therefore, from an alloy capable of operating at 540.degree. C. to 600.degree. C. is a major leap forward. Not only has it been a requirement for alloys of the present invention that they be capable of operating at service temperatures of up to 600.degree. C. but also the alloys have to meet operating requirements previously not considered important. Experience with operating aircraft engines has shown that the titanium alloy has to have resistance to such problems as stress rupture and low cycle fatigue in addition to all of the normal requirements of a high tensile strength, a resistance to conventional fatigue, ductility, stability, resistance to oxidation, a high creep resistance, forgability, weldability and many other properties.
In addition to changes to alloy compositions a great deal of work is being carried out to improve the properties of titanium alloys by modifying the heat treatment of the alloy. Titanium alloys of the high creep strength type are not used in the cast or forged condition but are given a series of heat treatments to modify and improve their mechanical properties. In part the present invention arises from the unexpected discovery that the presence of a certain element, namely carbon, in the alloys alters the shape of the alpha plus beta approach curve to make it practicable to work and heat treat the alloy in the alpha plus beta field. By way of explanation it is noted that titanium normally exists in two crystallographic phases, alpha and beta. The alpha phase, which is a close packed hexagonal structure, on heating, transforms at approximately 880.degree. C. in pure titanium metal to a body centre cubic beta phase, which is stable up to the melting point of the metal. Certain elements, known as alpha stabilisers, stabilise the alpha form of titanium such that the transformation temperature for such alloys is increased above 880.degree. C. By contrast beta stabilising elements depress the transformation temperature to below 880.degree. C. In alloys, as opposed to the pure metal, the transformation from alpha to beta on heating the alloy does not take place at a single temperature but the transformation takes place over a range of temperatures at which both the alpha and beta phases are stable. As the temperature increases the proportion of alpha decreases and the proportion of beta increases.
It has unexpectedly been found that small quantities of carbon leads to a significant change in the shape of the approach curve of the alpha plus beta phase proportions and furthermore the present invention provides a near alpha titanium alloy which, for the first time, can be not only fusion welded but is usable when it has been thermo-mechanically processed in either beta, alpha plus beta or beta plus silicide fields. Thus the present invention not only provides an alloy capable of being used in the alpha beta heat treated condition but also has transformation characteristics so as to make alpha beta heat treatment a practical proposition.
All compositions as used in this specification are expressed in terms of weight percentage. Thus all percentages as used herein will be weight percentage unless specifically indicated otherwise.